Log Invalid Level as an Error in SignalStrength

If one of the CellSignalStrength classes returns a
level value that's out of range, it should be logged
as an error. This CL adds loge() to SignalStrength
and logs this event at the correct level.

Bug: 8675309
Test: compilation
Change-Id: Ic4bd706ae83e647cd782a60d94db7077c6d0fc1b